One important performance attribute of an antenna is the antenna's radiation efficiency. One way to define or identify an antenna's radiation efficiency is as a ratio of the power radiated from the antenna to the power input to the antenna. The ratio indicates losses in the antenna system. Formula (1), below, defines radiation efficiency η of an antenna as:
                    η        =                              P            R                                              P              R                        +                          P              L                                                          (        1        )            where PR is the radiated power, and PL is the power loss of the antenna. The quantity PR+PL is the power input to the antenna. Because power input to the antenna is known, and power radiated from the antenna is measurable, power loss is derivable.
An equivalent definition of efficiency is through a ratio of radiation resistance and loss resistance. In this case, formula (2) measures antenna efficiency η as:
                    η        =                              R            R                                              R              R                        +                          R              L                                                          (        2        )            where RR is the radiation resistance, and RL the loss resistance. The quantity RR+RL is the input resistance of the antenna and it constitutes the real part of the antenna input impedance.
As is commonly understood in the art, the radiation resistance of an antenna is equivalent to the resistance of an equivalent ideal resistor which when replaced would deliver the same power as that of the antenna.
Conventionally, a 3D-Pattern Integration Method is used to determine the radiation efficiency of the antenna. The 3D-Pattern Integration Method of efficiency measurement is based on power, see formula (1), and involves measuring the radiation pattern over a sphere that encloses the test antenna.
While the conventional and accepted method of determining radiation efficiency, the 3D-Pattern Integration Method has several drawbacks. Some of the drawbacks are that the test is relatively expensive and time consuming. The 3D-Pattern Integration Method computes the efficiency of the antenna based on the gain computations, which in turn are determined through volumetric or 3D patterns of the test antenna.
An alternative to the 3D-Pattern Integration Method above involves a process conventionally known as the Cavity Method of Efficiency. The Cavity Method of Efficiency is also known as the Wheeler Method. This method originally proposed by Wheeler [H. A. Wheeler, “The Radiansphere around a Small Antenna”, Proceedings of the IRE, August, 1959, pp. 1325–1331] enables the determination of the antenna efficiency defined in terms of radiation and loss resistance, see formula (2) above. The Cavity Method of Efficiency assumes radiation resistance gets shorted when an antenna under test is enclosed by a conducting cavity. In other words, the antenna will not radiate when a conducting cavity encompasses, bounds, or shields the antenna. Under this assumption, the input resistance of the test antenna placed inside a shielded cavity is a direct measure of the loss resistance of the antenna. Thus by making two impedance measurements of the test antenna, one when the test antenna is in free space (to measure RR+RL) and the other when the test antenna is inside the shielded cavity (to measure RL), the radiation resistance RR can be determined because RR=(RR+RL)−RL. Once the radiation resistance RR and the loss resistance RL of the antenna are known, one can determine the antenna efficiency using formula (2).
When compared to the 3D-Pattern Integration Method, the Cavity Method of Efficiency is considered simpler, less time consuming and less tedious. However, it has its own drawbacks despite all these novel features. In particular, the Cavity Method of Efficiency has constraints on the bandwidth of the measurement. This means, depending upon the size of the cavity or shielding, the antenna may encounter an anti resonance of the cavity.
The term anti resonance defines that resonant frequency of the cavity which overlaps the measurement frequency of the test antenna. The measurement cavity has inherent different resonant frequencies depending upon the dimensions of the cavity.
The anti resonance of the cavity results in the measurement of loss resistance RL whose magnitude is higher or much higher than the input resistance (RR+RL) of the test antenna in free space. This in turn leads to a case of negative radiation resistance RR and therefore negative efficiency of the antenna. Both the negative radiation resistance RR and negative efficiency η of the antenna are unrealistic and defy the physical significance or meaning of the underlying antenna parameters.
In the past, the only recourse to overcome the anti resonance of the cavity was to use cavities of different sizes for different test antennas. Even for the same test antenna, there may be need to use separate cavities of varying sizes for different frequency bands of interest. The above-cited limitations have restricted the practical utility of Cavity Method of Efficiency. Thus, it would be desirous to develop a cavity that addressed the drawbacks of the existing Cavity Method of Efficiency.